The present invention relates to elevator systems, and more particularly to tension members for such elevator systems.
A conventional traction elevator system includes a car, a counterweight, two or more ropes interconnecting the car and counterweight, a traction sheave to move the ropes, and a machine to rotate the traction sheave. The ropes are formed from laid or twisted steel wire and the sheave is formed from cast iron. The machine may be either a geared or gearless machine. A geared machine permits the use of higher speed motor, which is more compact and less costly, but requires additional maintenance and space.
Although conventional steel ropes and cast iron sheaves have proven very reliable and cost effective, there are limitations on their use. One such limitation is the traction forces between the ropes and the sheave. These traction forces may be enhanced by increasing the wrap angle of the ropes or by undercutting the grooves in the sheave. Both techniques reduce the durability of the ropes, however, as a result of the increased wear (wrap angle) or the increased rope pressure (undercutting). Another method to increase the traction forces is to use liners formed from a synthetic material in the grooves of the sheave. The liners increase the coefficient of friction between the ropes and sheave while at the same time minimizing the wear of the ropes and sheave.
Another limitation on the use of steel ropes is the flexibility and fatigue characteristics of steel wire ropes. Elevator safety codes today require that each steel rope have a minimum diameter d (dmin=8 mm for CEN; dmin=9.5 mm (xe2x85x9cxe2x80x3) for ANSI) and that the D/d ratio for traction elevators be greater than or equal to forty (D/dxe2x89xa740), where D is the diameter of the sheave. This results in the diameter D for the sheave being at least 320 mm (380 mm for ANSI). The larger the sheave diameter D, the greater torque required from the machine to drive the elevator system.
With the development of high tensile strength, lightweight synthetic fibers has come the suggestion to replace steel wire ropes in elevator systems with ropes having load carrying strands formed from synthetic fibers, such as aramid fibers. Recent publications making this suggestion include: U.S. Pat. No. 4,022,010, issued to Gladdenbeck et al.; U.S. Pat. No. 4,624,097 issued to Wilcox; U.S. Pat. No. 4,887,422 issued to Klees et al.; and U.S. Pat. No. 5,566,786 issued to De Angelis et al. The cited benefits of replacing steel fibers with aramid fibers are the improved tensile strength to weight ratio and improved flexibility of the aramid materials, along with the possibility of enhanced traction between the synthetic material of the rope and the sheave.
Even ropes formed from aramid fiber strands, however, are subject to the limitations caused by the pressure on the ropes. For both steel and aramid ropes, the higher the rope pressure, the shorter the life of the rope. Rope pressure (Prope) is generated as the rope travels over the sheave and is directly proportional to the tension (F) in the rope and inversely proportional to the sheave diameter D and the rope diameter d (Prope≈F/(Dd). In addition, the shape of the sheave grooves, including such traction enhancing techniques as undercutting the sheave grooves, further increases the maximum rope pressure to which the rope is subjected.
Even though the flexibility characteristic of such synthetic fiber ropes may be used to reduce the required D/d ratio, and thereby the sheave diameter D, the ropes will still be exposed to significant rope pressure. The inverse relationship between sheave diameter D and rope pressure limits the reduction in sheave diameter D that can be attained with conventional ropes formed from aramid fibers. In addition, aramid fibers, although they have high tensile strength, are more susceptible to failure when subjected to transverse loads. Even with reductions in the D/d requirement, the resulting rope pressure may cause undue damage to the aramid fibers and reduce the durability of the ropes.
The above art notwithstanding, scientists and engineers under the direction of Applicants"" Assignee are working to develop more efficient and durable methods and apparatus to drive elevator systems.
According to the present invention, a tension member for an elevator has an aspect ratio of greater than one, where aspect ratio is defined as the ratio of tension member width w to thickness t (Aspect Ratio=w/t).
A principal feature of the present invention is the flatness of the tension member. The increase in aspect ratio results in a tension member that has an engagement surface, defined by the width dimension, that is optimized to distribute the rope pressure. Therefore, the maximum pressure is minimized within the tension member. In addition, by increasing the aspect ratio relative to a round rope, which has an aspect ratio equal to one, the thickness of the tension member may be reduced while maintaining a constant cross-sectional area of the tension member.
According further to the present invention, the tension member includes a plurality of individual load carrying ropes encased within a common layer of coating. The coating layer separates the individual ropes and defines an engagement surface for engaging a traction sheave.
As a result of the configuration of the tension member, the rope pressure may be distributed more uniformly throughout the tension member. As a result, the maximum rope pressure is significantly reduced as compared to a conventionally roped elevator having a similar load carrying capacity. Furthermore, the effective rope diameter xe2x80x98dxe2x80x99 (measured in the bending direction) is reduced for the equivalent load bearing capacity. Therefore, smaller values for the sheave diameter xe2x80x98Dxe2x80x99 may be attained without a reduction in the D/d ratio. In addition, minimizing the diameter D of the sheave permits the use of less costly, more compact, high speed motors as the drive machine without the need for a gearbox.
In a particular embodiment of the present invention, the individual ropes are formed from strands of non-metallic material, such as aramid fibers. By incorporating ropes having the weight, strength, durability and, in particular, the flexibility characteristics of such materials into the tension member of the present invention, the acceptable traction sheave diameter may be further reduced while maintaining the maximum rope pressure within acceptable limits. As stated previously, smaller sheave diameters reduce the required torque of the machine driving the sheave and increase the rotational speed. Therefore, smaller and less costly machines may be used to drive the elevator system.
In a further particular embodiment of the present invention, a traction drive for an elevator system includes a tension member having an aspect ratio greater than one and a traction sheave having a traction surface configured to receive the tension member. The tension member includes an engagement surface defined by the width dimension of the tension member. The traction surface of the sheave and the engagement surface are complementarily contoured to provide traction and to guide the engagement between the tension member and the sheave. In an alternate configuration, the traction drive includes a plurality of tension members engaged with the sheave and the sheave includes a pair of rims disposed on opposite sides of the sheave and one or more dividers disposed between adjacent tension members. The pair of rims and dividers perform the function of guiding the engagement of the tension member with the sheave.
In another embodiment, the traction drive includes a guidance device disposed proximate to the traction sheave and engaged with the tension member. The guidance device positions the tension member for proper engagement with the traction sheave. In a particular configuration, the guidance device includes a roller engaged with the tension member and/or the sheave to define a limited space for the tension member to engage the sheave.
In a still further embodiment, the traction surface of the sheave is defined by a material that optimizes the traction forces between the sheave and the tension member and minimizes the wear of the tension member. In one configuration, the traction surface is integral to a sheave liner that is disposed on the sheave. In another configuration, the traction surface is defined by a coating layer that is bonded to the traction sheave. In a still further configuration, the traction sheave is formed from the material that defines the traction surface.
Although described herein as primarily a traction device for use in an elevator application having a traction sheave, the tension member may be useful and have benefits in elevator applications that do not use a traction sheave to drive the tension member, such as indirectly roped elevator systems, linear motor driven elevator systems, or self-propelled elevators having a counterweight. In these applications, the reduced size of the sheave may be useful in order to reduce space requirements for the elevator system. The foregoing and other objects, features and advantages of the present invention become more apparent in light of the following detailed description of the exemplary embodiments thereof, as illustrated in the accompanying drawings.